In recent years, thermoplastic elastomers (hereinafter referred to as "TPE" for simplicity) having excellent flexibility, heat distortion resistance, and mechanical strength, and which is capable of molding like thermoplastic resins without the vulcanization step have received increasing attention in the fields of automobile parts, electric appliance parts, electric wire coatings, footwear, miscellaneous goods, etc.
Various types of polymers such as polyolefin-, polyurethane-, polyester-, polystyrene-, and polyvinyl chloride-based polymers are used as the TPE.
Of those TPE, polystyrene-based TPE such as a styrene/butadiene block copolymer (SBS) and a styrene/isoprene block copolymer (SIS) have an excellent flexibility and a good mechanical strength, and can also provide TPE compositions having an excellent workability. Those polymers, however, have a poor heat aging resistance (heat stability) and weatherability due to the fact that they contain double bonds as conjugated diene blocks in the molecule.
An elastomer having an improved heat stability can be obtained by hydrogenating double bonds in the molecule of the styrene/conjugated diene block polymers. Processes for producing such hydrogenated block copolymers are described in, for example, Japanese Patent Publication Nos. 8704/67, 6636/68, 20504/70 and 3555/73. In addition, TPE compositions using those hydrogenated block copolymers have been proposed. For example, Japanese Patent Application (OPI) Nos. 14742/75 and 65551/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") disclose compostions comprising hydrogenated styrene/conjugated diene block copolymers and hydrocarbon oils or .alpha.-olefin polymers.
The TPE compositions using the hydrogenated products of styrene/conjugated diene block copolymers, however, are not sufficiently satisfactory in flowability in the production of a large-sized exterior body part for an automotive vehicle, such as bumpers and facers, by injection molding. In order to improve such a poor flowability, it may be considered to increase the amount of a softening agent added. If, however, the amount of the softening agent added is increased, the mechanical strength is decreased. In particular, the scratch resistance is seriously decreased. Thus, the final exterior body parts tend to be easily scratched and their appearance is deteriorated.
An attempt has been made to improve such poor flowability and scratch resistance by adding a propylene homopolymer or a propylene/ethylene random copolymer. In this case, however, the impact strength at low temperatures is not sufficient. Thus, the resulting compositions are unsuitable for use, in particular, in the production of an exterior body part for an automotive vehicle.